Light

So far when we have talked about waves we have talked about sound waves. Light is a special type of wave.

Differences Between Sound Waves and Light Waves

- Sound waves need matter (gas, liquid, or solid) to vibrate in order to move from one place to the next. Light does not. It can travel through a vacuum.

- A sound wave tends to spread out in all directionsfrom its source where light wave travel in straightpaths

Why do sound waves spread out while light wave travel in straight paths?

-Scientists agree that light has a dual nature,-it is part wave and part particle

- A light particle is called a photonPhoton – massless bundles of concentrated

electromagnetic energy-This explains why light can pass though some things and not others. The fact that it is part wave allows it to travel through some things. The fact that it is part particle means that it can not travel through all things.

The Speed of Light

Light travels very fast (much faster than sound)- this is why you may see a jet fly over head going faster than the speed of sound but not hear it until it is well past you. The light gets to your eyes allowing you to see the jet before the sound waves get to your ears allowing you to hear the jet.

http://www.youtube.com/watch?v=7NJz_MOnu3A&feature=related

Speed of Sound Vs Speed of Light

Sound travels at 340 m/s where light is much faster.

Light travels at 300,000 km/s-Light from the sun takes 8 minutes to reach Earth.

How far is the Earth from the sun?v = distance / time 8 minutes = 480s300,000 km/s = distance / 480sDistance = 144000000km

Electromagnetic Waves

Energy travels in a wave that is partly electric and partly magnetic.

Electromagnetic Spectrum – the range of electromagnetic waves

Types of Electromagnetic Waves

Infrared – electromagnetic waves of frequencies lower than the red of visible light-Infrared lamps are used to keep food

warm at fast food restaurants

Types of Electromagnetic Waves

Ultraviolet – electromagnetic waves of frequencies higher than those of violet

- this is the light given off from the sun that you can not see but burns you

Light can go through some materials and not others

Transparent – materials that transmit light- light passes through these materials because their atoms absorb the energy and immediately reemit it as light

Light can go through some materials and not others

Opaque – materials that absorb light without reemission and thus allow no light through

-Opaque materials give off a shadow whenlight is shined on them

What happens when light hits an object?

Three things can happen when light hits an object;

1. It can get absorbed2. It can pass through3. It can get reflected off the object

When light gets absorbed by an object

When light gets absorbed by an object that object heats up because it is absorbing the energy from the light.

- The colors that object appear to us happen because the object absorbs all the colorsbut the one we see. The color we see is reflected to our eyes.

White Vs Black

When we see something that is white all colors are reflected back to our eyes.-when we see all the colors of the visible light spectrum at once we see white.

When we see something that is black all colorsare absorbed by the object and none arereflected to our eyes.

Clothing for the summer

On a sunny day a white shirt feels much cooler than black.-black absorbs all of the visible light and its energy so it is warmer-white reflects all of the visible light and their energy so it feels cooler

Why is the Sky Blue?The sky is blue because particles in the

atmosphere scatter high frequency light waves.

This scattering of light waves causes many high frequency colors of light (purple and blue) to be trapped in the atmosphere so that is what we see.

Why are sunsets orange and red?

By the time light gets to the ground at sunset all the high frequency light gets scattered.

All the low frequencies of light make it to the ground and that is why we see orange or red.

Why are the oceans blue?

Water molecules absorb red light and reflect blue. That is why we see large bodies of water as blue.

When light passes through an object

When light passes through an object that light was passing though air.

When it passes through another material such as glass, that light bends or refracts.

Refraction: the bending of a wave as it crosses the boundary between two media at an angle

RefractionThe reason that light bends or refracts is

because when it passes into a new material that material has a different density or number of atoms per area.

Incident ray – the ray of light as it is entering a new material

Normal – a line perpendicular to the surface ofthe new material

RefractionAngle of Incidence: the angel between the

incidence ray and the normalRefraction Ray: The ray of light that is refracted

as it passes through a new substance

Angle of Refraction – the angles between thenormal and the refracted ray

RefractionEmergent Ray – the ray of light as it exits or

emerges from a new substance

When light passes through a new substance itgets refracted. This is why an object in water is not where it may appear to bewhen you are looking at it outside of the water.

Special cases when you refract light.

Some objects refract light just the right amount as it passes through that it separates white light into its individual colors.

A prism, or rain drops separate white light into the individual colors so you can see all colors when light passes through.

This is how a rainbow works.On a rainy day when the sun is still shining light

passes from the air through the rain drops. When light does this it get refracted enough to see all the colors in white light making a rainbow.

Will you ever see the sun in a picture of a rainbow?

Reflection

Reflection – the return of a wave back to its original medium

Angle of Reflection – the angle between the incident ray and the normal

Reflection

Law of Reflection – the angle of incidence and angle of reflection are equal to each other.

Mirrors

Mirrors reflect light so you can see the object infrom of the mirror.

Virtual Image – an image that appears to be in a location where light does not really reach

Different Types of Mirrors

Plane Mirror – a mirror that produces only virtual images- what ever is in front of a plane mirror is what you see.- the object will appear as far away in a plane mirror as it is standing from that mirror

Convex Mirrors

A convex mirror is bent away from the object in front of it. It makes the object look smaller than it really is.

Concave Mirrors

A concave mirror is curves out at the ends toward the object looking at it. A concave mirror makes objects look larger then the really are.

Lenses

Lens – a piece of transparent material, such as glass, that refracts light as it passes through

-Lenses are used in all types of opticaldevices such as telescopes, cameras,microscopes, scopes, binoculars,…

Different Types of Lenses

Lenses can be used to look at things that are far away or at thing that are too small to see with the naked eye.

Different Types of Lenses

Converging Lens or Convex Lens – a lens that is thicker in the middle then at the ends, causing rays of light that are initially parallel to meet at a single point (called a focal point)

Different Types of Lenses

Divergent Lens or Concave Lens – a lens that is thinner in the middle than on the sides, causing the rays of light to appear to originate from a single point.

Principal Axis – a line drawn through the centers of curvature of a lens

Focal Point – for a converging lens is the point at which a beam of light parallel to the principal axis converges.

Focal Plane – a plane perpendicular to the principal axis that passes through either focal point of a lens.

Images formed by converging lenses

Real Image – an image formed by a single converging lens

-real images are formed through a single convergent lens are always upside down or inverted

-this is how movies are projected at the movie theater

-the image after going through the convergent lens is always larger.

Images formed by diverging lenses

When you look through a divergent lens the image that you look at is always smaller.

-divergent lenses are use as view finders in a camera